1. Field of the Invention
The present invention relates to a method for driving a piezoelectric actuator which is foraged of a piezoelectric effect element.
2. Description of Related Art
One typical piezoelectric actuator, called a "multilayer piezoelectric ceramic actuator", includes a piezoelectric effect element formed by stacking a number of thin sheets of piezoelectric material in a displacement direction, so that a large amount of displacement can be obtained with a low driving voltage. An internal electrode is interposed between each pair of adjacent piezoelectric material sheets, and the internal electrodes are interconnected every other one, so that all the internal electrodes are divided into two groups. The stacked piezoelectric material sheets are encapsulated with a resin casing in such a manner that opposite end surfaces of a stacked body of piezoelectric material sheets are exposed but a side surface of the stacked body is completely covered by the resin casing.
If the above mentioned piezoelectric actuator is used an atmosphere of high humidity, moisture contained in the atmosphere gradually enters the resin casing over a period of time, and ultimately, reaches the surface of the stacked body of piezoelectric material sheets so that moisture condensation or sweating occurs on the surface of the stacked body in the inside of the resin casing.
For example, each of the piezoelectric material sheets has a thickness of about 100 .mu.m, and a voltage of 150 V is applied between two groups of internal electrodes. Namely, 150 V is applied across each piezoelectric material sheet. This means that an intense electric field of 150000 V/cm is applied. Due to this intense electric field and the moisture condensation or sweating on the surface of the stacked body, an electric discharge will occur between the internal electrodes or between the internal electrode and an internal wiring formed between the side surface of the stacked body and the resin casing. As a result, the piezoelectric actuator breaks down or destroyed.